The problem with the monoplot is that everything is connected to it. If the PCs encounter a tomb that has been defiled, the monoplot defiled it. A barony that has been taken over by bandit thugs? Sent by the monoplot; and someone there will help with the monoplot. Someone poisoned the river? Done by the monoplot villain. Artifact in a dungeon? Use it to solve the monoplot. And so on. It gets to the point where the players don't have to think about what's going on. The monoplot is what's going on. Everything monoplot.
This kind of super-tight story is ideal for one-shots and fine for single adventurers, but it can be dangerous to long campaigns. It can cause chronic time pressure, which is bad for pacing and lead to a railroaded feel. The player characters' personal hooks to the monoplot eventually resolve, leaving them plodding forward trying to save the world only because nobody else seems interested in taking over. And the player-characters' personal hooks that didn't connect to the monoplot might get ignored.
When you have monoplot, if the PCs discover a group planning to assassinate the king, they can guess who's behind it. It's always going to lead back to the big bad villain of the monoplot. When there are two or more major plots in your campaign, they don't know. Who's behind it? Villain A? Villain B? Villain C? They're kept guessing, and that motivates them to look for clues. That draws them into the action much better.
Here are four tips to fight off monoplot:
1. Write another villain.
Too often, a GM designs just one big bad villain. That villain may have several henchmen doing various things to modulate your players' interest, but all of them lead back to the monoplot.
The solution is to write another big bad villain. Villains always want something. Make this villain want something totally different from what the first wants. It's best if your second villain is pulled directly from one of your PCs' back-story and personal plot hooks. This draws that player deeper into the fiction.
Say your first villain is the Wyrm that Eats the World, a long forgotten god that is returning to bring about an apocalypse. Then perhaps the second could be a paranoid empress who wants to trick Eastwatch and Landing, two city-states bordering on her lands, into war with each other to keep them weak and dependent on her grain and ore, and to prevent either from invading her lands. Sure, it's not as big a deal as saving the world, but one of the PCs has a home, family and allies in Eastwatch, and another has a mentor in Eastwatch. When they learn that all of that is at risk because of a mad empress, they will want to stop it.
2. Put on the brakes
Solve your chronic time pressure by creating breaks in your monoplot for other things to happen. When the big bad villain steals the artifact of awesomeness, your players will want to chase him down immediately. If you want to avoid a monoplot situation, you need to step on the brakes. Maybe the villain fled with a teleportation scroll, and the PCs need to go back to the city to get a powerful diviner or well-connected spymaster to find out where he teleported to. This process will take a month, giving the PCs time to explore another plot.
Here are some ways to pause the plot:
- Have to rely on an NPC to find the next step in the plot. The NPC will tell them how long it will take to find it. Maybe it's a merchant whose ships bring back news from around the world. Maybe it's a druid searching for an omen at the next full moon. Maybe it's a wizard studying an artifact the PCs brought out of a dungeon. Maybe it's a sage translating an ancient scroll. Maybe the PCs captured one of the villain's henchmen, but can't get him to talk, so they hand him over to nearby clerics to use Detect Lie and Zone of Truth and other such spells to try to draw the next plot lead out of him. He is strongly magic resistant, and quite canny, so it will be some time before he lets valuable intelligence slip out.
- Travel to the next place in the plot is prevented somehow - medieval folks found travel over mountains to be nearly impossible; same with crossing dangerous seas during typhoon season; maybe there's a desert that can't be crossed without an airship; etc.
- On the other hand, travel could be underway. Say the PCs know the next step in the Wyrm that Eats the World plot is in a city a thousand miles away. Along their journey, they can learn that the mad empress has built a training camp for her spies in the swamp near a fishing village on their route. They can afford to take an extra day to try to capture the empress' spymaster.
- The PCs could play a little henchman whack-a-mole: Let them totally defeat one of the big bad villain's lieutenants, crushing her organization utterly. They know there are other henchmen out there, but they have no news, yet, of one of their evil schemes. But it's only a matter of time!
3. Kill two birds with one stone
When you're designing a megadungeon, site-based adventure, hex crawl or some other kind of modular, open-world segment of the campaign, consider dropping hooks and challenges for two or more main plots in the location the PCs are exploring. Perhaps as the PCs rush to the ruins of a village in a valley outside Eastwatch to investigate the supposed war crimes of the King of Landing, they'll find the entrance to a newly-carved subterranean temple dedicated to the Wyrm that Eats the World. Of course, that temple points to the fact that the valley was peppered with three ancient ruins dedicated to the Wyrm that the cultists are searching for. Plus, you can seed PCs' unrelated plot hooks in too! Now they have...
- Some of the empress' scouts hiding in the valley to make sure their ruse worked,
- Work to do to help rebuild the village (refugees to reunite, lost children to find, monsters to kill to re-open dangerous woods for logging, neighboring villages to do favors for to get much needed building material, ghosts of murdered villagers to put to rest, etc.),
- Village elders to placate so they don't blame Landing,
- One PC's mentor who was killed in the attack, who left behind a puzzling will,
- Suspicious villagers to investigate to find the empress' leave-behind spies and secret Wyrm cultists,
- A subterranean temple to explore, and
- Three ancient ruins to find in the valley before the cultists find them.
4. Watch the progress bar fill up
Make the next big step in your monoplot something pretty mundane: Building something. The PCs must return to their home base and employ craftsmen or wizards or priests to make something that they need. Maybe they need to build a fortification to protect a city, or erect a massive monolith to power a magical ritual, or a computer capable of hacking Arasaka's mainframe (I'm using mostly fantasy examples, but, see, this advice works for other genres, too!), or forge a sword capable of slaying the Wyrm that Eats the World itself. The PCs will do relatively little of the work -- adventurers are great at skirmishes and stealth missions, but they employ experts for careful craftsmanship.
For example, the PCs must use the airship design plans they found to build an airship before the Wyrm cultists can build theirs. During the process, there is likely to be quite a lot of downtime -- it could take a year for a shipyard to construct a Georgian man of war, so you can easily stretch weeks if not months of downtime between the small adventures involved in the airship building process. During those downtimes, they can investigate spies in the Eastwatch court sent by the mad empress, and maybe try to turn one of them to their side. This way they're moving as fast as they can on the apocalypse cult plot while also pursuing the mad empress plot and some of their personal plot hooks.